Since an organic electroluminescence device (hereinafter, electroluminescence sometimes abbreviated as “EL”) of the laminate type driven under a low electric voltage was reported by C. W. Tang of Eastman Kodak Company (C. W. Tang and S. A. Vanslyke, Applied Physics Letters, Volume 51, Pages 913, 1987), many studies have been conducted on devices using organic materials as the constituting materials. An organic EL device is an emitting device having at least an organic emitting layer interposed between at least one pair of electrodes. The organic EL device is a spontaneous emitting device that utilizes the following principle: a fluorescent substance emits light with the recombination energy of a hole injected from an anode and an electron injected from a cathode when an electric field is applied to the substance. The organic EL device has various characteristics including the following ones: the device can emit light with high efficiency, is available at a low cost, and has a light weight and a small thickness.
The following phenomenon has been known as a problem inherent in the organic EL device: the luminance of the device reduces over time in association with the driving of the device. Various improvements have been attempted to suppress the reduction in luminance.
One known cause for the reduction in luminance is the presence of various impurities included in organic compound layers including the organic emitting layer as a component for the organic EL device, and the following attempt has been reported: the emission half lifetime of the organic EL device is improved by reducing the amounts of the impurities.
Examples of the impurities include: an impurity included at the time of the washing of the substrate of the organic EL device (see Patent Document 1); an impurity such as adsorbed water vapor or an adsorbed gas (see Patent Document 2); a halogen-containing compound (see Patent Documents 3 to 5); a substituted carbazole compound having an amino group (see Patent Document 6); a metal ion (see Patent Document 7); and a solvent having a large dipole moment (see Patent Document 8).
None of the above respective reports discloses that the emission half lifetime of the organic EL device reduces when even a trace amount of a compound having a hydroxyl group, in particular, a hydroxyl group-containing compound as a precursor or by-product which: has substantially the same basic skeleton as that of an organic compound material of which the organic emitting layer is constituted; and is expected to function as an organic compound for the device is included as an impurity and that the emission half lifetime is improved by reducing the amount of such impurity.
In particular, the hydroxyl group-containing compound as a precursor or by-product has a structure similar to that of the organic compound material. Accordingly, it is often difficult to separate the hydroxyl group-containing compound from the organic compound material to purify the organic emitting layer, and no attention has been paid to the hydroxyl group-containing compound despite the potential of the compound to serve as an impurity that adversely affects the emission half lifetime.
An influence of such impurity will be specifically described below. Upon synthesis of an organic compound for an organic EL device, a hydroxyl group remains in the compound in some cases. When a synthesis route for the compound includes the step of producing a compound containing a hydroxyl group as a precursor, the final reaction compound inevitably contains a considerable amount of a hydroxyl group-containing compound as a precursor as it is or as a by-product. Once the hydroxyl group-containing compound as a precursor or by-product is included in the main product, there arises the following problem: the hydroxyl group-containing compound cannot be easily removed unless the compound is intentionally removed because the compound and the main product have the same skeleton structure or similar skeleton structures. The inventors of the present invention have made extensive studies, and, as a result, have found that a hydroxyl group-containing impurity out of multiple possible impurities extremely affects the emission lifetime of the organic EL device. When a hydroxyl group-containing compound is produced as a precursor or by-product for the convenience of the synthesis route, the final reaction compound contains a large amount of the hydroxyl group-containing compound, so the problem becomes remarkable.
Here, in a technique of an organic EL field, the presence of an impurity and the performance of a device are complicatedly related to each other. For example, the following story behind the fact that an organic EL technique has reached a practical level is well known: the luminous efficiency and lifetime of an organic EL device are drastically improved by doping a host material with a specific impurity. However, even in such doping technique, when the concentration at which the host material is doped with the impurity deviates from an optimum range, the concentration quenching of the device or the deterioration of the lifetime of the device occurs. On the other hand, the presence of a certain kind of an impurity is known to exert, for example, the following adverse effect: the impurity deteriorates the color purity of the device, increases the voltage at which the device is driven, or deteriorates the lifetime of the device.
Although it has been known that, as described above, the addition or removal of a certain kind of an impurity is important in the organic EL field, it is extremely laborious and extremely costly to manage the concentrations of all impurities that may be present appropriately, and the management has presented a large obstacle to the full-fledged commercialization of organic EL devices. In the present invention, an OH group-containing compound was specified as a target specific impurity the concentration of which was to be controlled.    [Patent Document 1] JP 10-255972 A    [Patent Document 2] JP 11-92915 A    [Patent Document 3] WO 00/041443 (JP 3290432 A)    [Patent Document 4] JP 2002-175885 A    [Patent Document 5] JP 2004-327454 A    [Patent Document 6] JP 2004-311415 A    [Patent Document 7] JP 2005-150099 A    [Patent Document 8] JP 2006-236629 A